This invention relates to an image processing system for performing spatial frequency processing for image data and outputting the resultant data and an image processing coefficient determination method and an image processing coefficient calculation system for the image processing system.
It is demanded to provide the same output image independently of output devices under the present circumstances wherein color images are edited and output by using an image processing system such as a desktop publishing (DTP) system to which a number of output devices are connected. However, the reproduction characteristics of image output devices vary largely from one device to another, and output images having the same characteristic cannot be provided if the images are output as they are.
For the variations in the reproduction characteristics of image output devices, the same color reproduction output image can be provided independently of the image output devices with respect to color reproduction. For example, a color management system (CMS) is used to execute color conversion from device-independent color signals to color signals proper to each device, whereby the same color reproduction output image can be provided independently of the output devices. Color conversion coefficients used for the color conversion can be calculated by measuring colors of a number of color patch output images, for example, as described in the Unexamined Japanese Patent Application Publication No. Hei 2-289367. The calculated color conversion coefficients are described in the ICC profile format, for example, as device characteristic information and are held in the system.
However, if image data to which color conversion has been applied in response to each output device is prepared and output, output images different in sharpness are produced for each output device. In the image processing field, a CTF (contrast transfer function) indicating how much contrast can be transferred for each frequency is used as a sharpness comparison scale. In the description to follow, the CTF is adopted as the spatial frequency characteristic of each output device.
Under the present circumstances, output image change caused by the effect of the spatial frequency characteristic of each output device is predicted qualitatively by intuition and experience and image processing application software is used to repeat spatial frequency processing such as sharpness processing and smoothing for image data until a desired output image is provided.
At the development stage of a color printer containing a spatial frequency processing section, processing coefficients of the spatial frequency processing section are determined. At the time, while the processing coefficients are changed based on intuition and experience, output images of different kinds of reference image data are prepared on the color printer and sharpness of the output images is observed for determining the processing coefficients.